Star
A star is a massive, self-luminous sphere of plasma held together by its own gravity. It shines by converting hydrogen into helium through nuclear fusion in its core, releasing energy across the electromagnetic spectrum.
Stars form in large clouds of gas and dust—called nebulae—where regions collapse under gravity, creating protostars. When core temperatures reach millions of degrees, fusion ignites, marking the birth of a star. Observatories like Hubble and missions such as NASA’s Infrared telescopes have imaged this process in action.
The majority (~90%) of stars are main-sequence stars, fusing hydrogen into helium. These include a broad range of masses—from red dwarfs (small, long-lived, faint) to blue giants and supergiants (massive, hot, and short-lived). Our Sun is a middle-aged G-type main sequence star.
As stars exhaust their hydrogen fuel, their evolution depends on mass. Lower-mass stars become red giants then white dwarfs. More massive stars undergo successive fusion stages, end in supernova explosions, and leave behind neutron stars or black holes.
Stars vary in brightness, size, and color. They are classified using spectral types (O, B, A, F, G, K, M) based on surface temperature and absorption lines. For example, O- and B-type stars are hot and blue; M-type are cool and red.
Stellar remnants include white dwarfs (Earth-sized cores of former stars), neutron stars (city-sized remnants of supernovae), and black holes (extreme-density objects from the most massive stars).
Stars are not static—many rotate, exhibit magnetic activity (like sunspots and flares), and broadcast stellar winds. Their lifecycle enriches the interstellar medium with heavier elements, seeding future generations of stars and planets.
Stars often exist in groups—binary or systems within star clusters and galaxies. Their properties are studied via brightness, spectra, parallax, variability, and statistical surveys by missions like Gaia and Kepler.
Sources
APODs including "Star"
Aurora Australis and the International Space Station
13 September 2024
This snapshot from the International Space Station was taken on August 11 while orbiting about 430 kilometers above the Indian Ocean, Southern Hemisphere, planet Earth. The spectacular view looks south and east, down toward the planet's horizon and through red and green curtains of aurora australis. The auroral glow is caused by emission from excited oxygen atoms in the extremely rarefied upper atmosphere still present at the level of the orbiting outpost. Green emission from atomic oxygen dominates this scene at altitudes of 100 to 250 kilometers, while red emission from atomic oxygen can extend as high as 500 kilometers altitude. Beyond the glow of these southern lights, this view from low Earth orbit reveals the starry sky from a southern hemisphere perspective. Stars in Orion's belt and the Orion Nebula are near the Earth's limb just left of center. Sirius, alpha star of Canis Major and brightest star in planet Earth's night is above center along the right edge of the southern orbital skyscape. Looking Up: International Observe the Moon Night